I came across this journal article by Chris Webster, a JPL scientist, whose lab is building a small laser spectroscopy unit to detect methane on Mars . The device is fairly small, and is sensitive enough to detect methane at concentrations well below current estimates for the Martian atmosphere.
But to identify methane as coming from life, the device needs to compare isotopes in the gas. Living things prefer to use carbon-12 over carbon-13, whereas geological processes don't distinguish between them (this is the same method used to search for biogenic traces in very old earth rocks. I didn't realize this but the enrichment is on the order of 2-4%. The device will also be able to measure oxygen and hydrogen isotopes.) To measure the ratio of these two carbon isotopes, the device needs much higher concentrations of methane. Perhaps this limitation could be bypassed if a vent or other big source of methane could be located.
Some technical info about the laser spectrograph is available here. Subscription pdf file (Applied Optics) is here.